1. Field of the Invention
The present invention pertains to the field of immunology and virology and, more particularly, to a vaccine derived from a pathogenic viral isolate. More specifically, the pathogenic form of the virus is modified or attenuated to an avirulent form according to methods of producing the vaccine for use against a devastating new swine disease.
2. Description of the Prior Art
A new swine disease, variously referred to as "mystery swine disease", "swine infertility and respiratory syndrome", "blue ear disease", or "porcine reproductive and respiratory syndrome" (PRRS) is causing heavy losses in breeding herds of the United States and Canada. A similar disease has also appeared in much of Europe. The disease is manifested in two forms, one causing reproductive failure and the other producing respiratory distress in young pigs. The reproductive form of the disease is described by Keffaber, K. K., "Reproductive Failure of Unknown Etiology", American Association of Swine Practitioners Newsletter, 1:109 (1989).
The most prominent clinical symptoms of the reproductive form of the disease are spontaneous late-term abortions, premature births (which can be as high as 20-30% of all births) and the farrowing of mummified fetuses, stillbirth or sickly piglets. Such clinical symptoms will typically be observed in a herd from 4-16 weeks, or even longer. Stillborn fetuses in affected litters often are in the early stages of mummification, as evidenced by tan-brown discoloration of the skin and post-mortem autolysis. Dome-shaped malformations of fetal skulls is also sometimes seen. The infection of sows may go unnoticed, or may manifest itself by an impaired general condition lasting up to a few days. For example, the sows may go off feed, and experience body temperatures either above or below normal. In the farrowing phase, the sows may exhibit depression, lethargy, phyrexia and occasional vomiting. In some affected herds, up to 75% of all piglets may be lost. The economic consequences of the disease, accordingly, are devastating.
The respiratory form of the disease exhibits clinical signs which are most pronounced in piglets of 3-8 weeks in age, but are reported to occur in pigs of all ages in infected herds. The diseased piglets grow slowly, have roughened hair coats, respiratory distress ("thumping") and increased mortality (up to about 80% pre-weaning mortality).
Findings in preliminary studies of gross and microscopic lesions of piglets affected with the respiratory form of the disease suggest that microscopic lung lesions are an important clinical feature of this disease. Despite pronounced respiratory symptoms of disease, lungs that appear uncomplicated by secondary bacterial infection are either grossly normal or have a mild, diffused tan-gray discoloration of the lung surface. Nevertheless, a microscopic examination of the lung tissue of PRRS-diseased piglets reveals a characteristic pattern of interstitial pneumonitis, per Collins, J. E. et al., "Respiratory Disease in a Swine Herd Experiencing a Reproductive Failure Syndrome", Proceedings, Minnesota Swine Conference for Veterinarians, p. 254, St. Paul, Minn. (Sep. 10-18, 1990).
Accordingly, there is a real need in the art for an effective vaccine which can eliminate or at least ameliorate the effects of PRRS.